The present invention relates generally to seat belt safety systems and particularly to safety systems having integrated belt pretensioners or belt tighteners.
If a vehicle collision occurs, injury to the vehicle occupant can be reduced through the use of safety belts which restrain the occupant and reduce the acceleration experienced by a vehicle seat occupant. Within the automotive industry, extensive testing has resulted in a series of design parameters which correlate maximum acceleration and jerk (acceleration per unit time) with injury thresholds. These parameters provide the basis for designing safety equipment such as seat belts and air bags, which limit maximum acceleration and jerk to levels which are survivable or less injurious to the occupant.
Safety belts are typically manufactured to deform and absorb energy under load. This deformation takes place by a stretching of the belt material when tension is applied. During a crash a vehicle occupant's body is decelerated by the seat belts, thereby restraining the forward motion of the occupant. Stretching of the belt absorbs a portion of the seat occupant's kinetic energy and allows the deceleration forces applied to the occupant's body to extend over a longer period of time. By increasing the time over which the deceleration forces are applied to the occupant, belt stretching decreases the levels of force applied to the occupant.
While belt stretching alone does reduce the acceleration forces experienced by a seat occupant, it is desirable to further reduce the acceleration forces by extending the period of time over which restraining forces act.
The duration of belt force action can be extended in two ways: first by applying restraining forces earlier in a collision, and second by permitting the restraining forces to extend for a longer time after the collision begins. Seat belts, however, do not begin to restrain the occupant until the occupant has been thrown forward into the belt. To rapidly remove belt slack, seat belt pretensioners have been developed which remove slack from the belts before the occupant moves significantly due to the crash-induced acceleration. While there are various techniques for extending the duration of the restraining forces after the collision, there is a limited range of forward travel before the occupant impacts some portion of the vehicle. Hence, seat belts must complete their task before the occupant's body travels too close to a portion of the vehicle, such as the dashboard, the steering wheel, or the windshield.
Safety belt pretensioners extend the deceleration time by removing slack from the belt early in a collision. Pretensioners typically remove slack by retracting a segment of belt before the occupant's body mass accelerates forward into the belt. Since the belt system is pulled tight around the seat occupant before the occupant's body can accelerate forward, the restraining forces begin to decelerate the occupant earlier in the collision.
Various types of pretensioners are known. For example, rotary pretensioners rapidly wrap a segment of belt onto a spool. Linear pretensioners, on the other hand, displace a segment of belt linearly along the belt path.
The pretensioner is typically located at one of the attachment points of a conventional three-point safety belt. The conventional safety belt comprises a lap belt and shoulder belt which attach to the vehicle at three points. The lap belt typically attaches at the outboard side of the vehicle seat, below and to the rear of the seat occupant. The end of the lap belt defines the first attachment point. The lap belt extends across the lap portion of the seat to the buckle. The buckle and associated tongue define the second attachment point. The shoulder belt typically is secured at the buckle (or slides through the buckle) and extends upward diagonally across the occupant's torso and over the occupant's shoulder, bends around a shoulder belt guide and extends downwardly to the third attachment point, typically the retractor. The belt guide may be mounted to the top portion of the vehicle seat or to the vehicle body structure at the B-pillar or C-pillar.
In the retractor, the belt is typically wound around a spool which rotates about an axis. When a collision occurs, the belt retractor will lock, preventing the seat belt from protracting from the spool. The retractor typically includes a vehicle sensor which causes the retractor to lock when the vehicle is subjected to decelerative levels above a pre-determined amount. The retractor is also locked by operation of a web sensor when the seat belt webbing is pulled from the spool at a level that exceeds a predetermined amount.
Pretensioners typically include a piston and cylinder assembly driven by a pyrotechnic device. The piston and cylinder assembly includes a combustion chamber, next to which the pyrotechnic gas generator is mounted. The pyrotechnic device includes a quantity of material which, when ignited, creates a volume of gas which is trapped within the combustion chamber. The pressure created by the entrapped gas pushes the piston along the cylinder. The rapid and forceful piston acceleration delivered by the pyrotechnic charge makes the pyrotechnic device an attractive power source for the pretensioner.
In a linear pretensioner, the piston is typically coupled to the end of the belt so that the piston pulls the buckle which pulls on the safety belt. The piston may alternatively pull a tongue or a clutch which pulls on the safety belt. While these arrangements transfer the fall piston force to the safety belt, the amount of slack that can be removed by the pretensioner is limited to the length of the piston stroke.
Slack removal can also be obtained with a rotary pretensioner. The rotary pretensioner typically comprises a retractor geared to a pyrotechnic piston and cylinder assembly. When a collision is detected, the retractor is locked, and then an electronic control unit sends a signal to activate the pyrotechnic piston and cylinder assembly. The pyrotechnic piston and cylinder assembly drives a gear which turns the spool. The turning of the spool rewinds the belt.
Some of the slack winding capacity of rotary pretensioning devices is lost to the "feed-roll" or "spool" effect. This occurs because the belt is typically wound somewhat loosely onto the spool. When tension is applied to the belt, the portion of belt on the spool tightens, releasing a segment of belt back into the belt system.
While the gearing of a rotary pretensioner does typically remove more slack than the linear pretensioner, mechanical limitations restrict the rate at which the belt can be retracted. The retraction force available through these devices is reduced by the gearing and further limited by the load capacity of the gear components.
What is needed is a seat belt safety system with a pretensioner which removes more slack from the safety belt system and provides a greater pretensioning force.
The seat belt safety system of this invention attains greater belt pretensioning by upward movement of a shoulder belt guide. Raising the guide extends two segments of belt, one segment extending frontwardly across the seat and one segment extending downwardly into the seat. The shoulder belt guide is mounted to a piston and cylinder assembly which is integrated into a vehicle seat frame. A pyrotechnical charge drives the piston and cylinder assembly. A signal from a crash detecting logic unit activates the pyrotechnical charge, which results in the production of high pressure gas. The gas, entrapped within the cylinder, pushes on the piston so that it extends upwardly from the cylinder. The piston extension raises the shoulder belt guide. The positioning of the belt over the guide provides a mechanical advantage. Due to the mechanical advantage, the amount of belt tensioning is greater than the linear displacement of the piston. Not only is the amount of belt slack removal increased, but the rate at which the belt is retracted is also increased proportionally. The ability to rapidly remove slack from the seat belt allows the seat belt to begin loading, that is decelerating the occupant, at the maximum rate early in the crash. The result is a greater ability to reduce crash induced accelerations below a design threshold which is correlated with serious injury.
After the piston is fully extended, a pair of wedges jams the side walls of the cylinder, preventing the piston from collapsing back into the cylinder. The wedges work as a "one way clutch," permitting upward piston movement while resisting downward piston movement. A row of teeth on the wedges provides extra "bite" on the cylinder wall, thereby amplifying the jamming effect.
It is a feature of the present invention to provide a seat belt safety system which provides for faster tensioning of a safety belt.
It is an another object of the present invention to provide a seat belt safety system which delivers greater pretensioning force.
It is a further feature of the present invention to provide a seat belt safety system which removes more slack from the safety belt.
It is a still further feature of the present invention to reduce the acceleration forces experienced by a vehicle seat passenger.
It is still another feature of the present invention to absorb energy during a vehicle collision.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.